The present invention relates to a surface acoustic wave filter and a surface acoustic wave resonator, and more particularly, to a device structure which accomplish a high Q-value (ratio of impedance at a resonant frequency and impedance at an anti-resonant frequency) and a low insertion loss in a resonator-type surface acoustic wave filter and a surface acoustic wave resonator for use in mobile communications and the like.
SAW devices which utilize surface acoustic waves (hereinafter called “SAW” in some cases) generated by the piezo-electric effect are widely used in recent years for resonators, filters, duplexers and the like because of their small sizes and light weights as well as suitability for higher performance.
Such a SAW device is generally created by forming a resonator by providing interdigital transducers (hereinafter called “IDT” in some cases) which opposes comb-shaped electrode having a plurality of interdigital electrodes arranged in a propagation direction of surface acoustic waves and bus bars for connecting these interdigital electrodes against each other, and has the interdigital electrodes so overlap as to insert with one another, and electrically or acoustically connecting this.
As connection structures for resonators, there are known a structure which has a plurality of IDTs arranged in a propagation path of surface acoustic waves for acoustic coupling (for example, a longitudinally coupled multi-mode SAW filter), a ladder structure which connects a plurality of resonators in a ladder shape, and the like. Also, attempts have been made to improve electric characteristics by providing a reflector at both ends of each resonator in order to trap surface acoustic waves, varying the geometry of IDT such as an electrode period, the number of electrode pairs, an overlap (insert) length and the like.
For example, the following patent documents disclose such SAW devices, where the following techniques have been created in inventions described in these documents, respectively:
Patent Document 1: JP-A-2005-295049;
Patent Document 2: JP-A-2002-314366;
Patent Document 3: JP-A-2003-309448;
Patent Document 4: JP-A-2005-159835; and
Patent Document 5: JP-A-11-225038.
Patent Document 1 (JP-A-2005-295049) improves an insertion loss and a bandwidth and prevents damages of IDT electrodes by repeatedly providing non-conductive portions and conductive portions at a fixed pitch in bus bars to reduce the velocity of SAW in a bus bar section, and also shifting the center of the conductive portion with respect to the center of an interdigital electrode of a comb-shaped electrode.
Patent Document 2 (JP-A-2002-314366) extends dummy electrodes from bus bars to reduce the distance to the leading end of interdigital electrodes, thereby suppressing SSBW (Surface Skimming Bulk Wave) and improve the flatness of a filter.
Patent Document 3 (JP-A-2003-309448) improves an insertion loss, a bandwidth, and VSWR (Voltage Standing Wave Ratio) in a SAW resonator connected in series to a longitudinally coupled resonator type filter by reducing the distance between a dummy electrode extended from a bus bar and an interdigital electrode, or making the length of the dummy electrode appropriate to shift spurious caused by SSBW.
Patent Document 4 (JP-A-2005-159835) provides a dummy electrode 224 between electrodes and bus bars of an IDT in a longitudinally coupled multi-mode filter so as to restrain radiations of surface acoustic waves in oblique directions. In this way, the SAW velocity outside of a main waveguide is made lower than a SAW velocity at an overlap zone of comb-shaped electrodes which are waveguides, and restrains oblique radiations of the surface acoustic waves, thereby improving an insertion loss and a bandwidth.
Patent Document 5 (JP-A-11-225038) prevents damages of IDT electrodes by connecting a plurality of electrodes from bus bars to interdigital electrodes.